5e Monster Catcher's Handbook

From publisher blurb:

For the first time, the Monster Catcher’s Handbook includes everything players and Game Masters need to collect, train, and battle with their own monstrous companions. Best of all, characters of all levels, classes and backgrounds can engage in monster catching! Also included are game statistics for over two hundred collectable creatures to populate your adventures, new magic items, and all-new random battle rules that can be used with—or without—a Game Master.